Added methods to use Sistrix API

Fixes issue #88.

If my other PR is merged, you can cherrypick the relevant commits from this PR. Thanks!
Took that code from [here](https://github.com/ClemensSahs/SEOstats/blob/hotfix/semrush/SEOstats/Services/SemRush.php#L221-227). Will remove it, can you remove it from your code too?